1. Field of the Invention
The present invention relates to a flat panel display device and more particularly, to an organic electroluminescent device and manufacturing method for the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been most widely used in the field of flat panel display devices due to their lightweight and low power consumption. The liquid crystal display (LCD) device is not a light emitting element but rather a light-receiving element that needs an additional light source to display images. Accordingly, there is a technical limit in improving brightness, contrast ratio, viewing angle and enlarging the size of a liquid crystal display panel. For this reason, mush research has been ongoing in this field to develop a new flat panel display element that can overcome the aforementioned problems.
The organic electroluminescent device is one of the those new flat panel display elements. Because the organic electroluminescent device emits light, allows for a wide viewing angle and a large contrast ratio, the organic electroluminescent device is superior when compared to the liquid crystal display (LCD) device. In addition, an organic electroluminescent device does not need a backlight. Further, an organic electroluminescent device also has the advantages of being light weight, a thin profile and low power consumption. Moreover, the organic electroluminescent device can be driven with a low DC (direct current) and has a fast response time. Because the organic electroluminescent device uses a solid material instead of a fluid material, such as liquid crystal, an organic electroluminescent device is more stable if it receives an external impact. The solid material also has a wider range of operating temperatures as compared to liquid crystal.
The organic electroluminescent device also has an advantage in terms of production costs as compared to an LCD device. Specifically, a deposition apparatus and an encapsulation apparatus are all that is needed for manufacturing an organic electroluminescent device, while the liquid crystal display (LCD) device or Plasma display panels (PDPs) need many kind of apparatus. Thus, the manufacturing process for the organic electroluminescent device is very simple compared to the liquid crystal display (LCD) device or the plasma display panels (PDPs).
Organic electroluminescent devices may be classified into either a passive matrix-type device or an active matrix-type device. In the active matrix-type organic electroluminescent device, voltage applied to the pixel is also stored in a storage capacitor CST such that the voltage is maintained until a signal for the next frame is applied. Accordingly, the pixel can retain the signal until the next frame regardless of the number of the scan lines. The active matrix-type organic electroluminescent device can obtain an appropriate luminance with low direct current (DC). Further, the active matrix-type organic electroluminescent device has advantages, such as low power consumption and high resolution with a large screen size. A basic structure and an operational property of the active matrix-type organic electroluminescent device will be described hereinafter with reference to FIG. 1.
FIG. 1 is a circuit diagram of a pixel of a related art active matrix organic electroluminescent device. A basic structure and an operational property of the active matrix-type organic electroluminescent device will be described hereinafter with reference to FIG. 1. As shown in FIG. 1, a scan line 2 is formed in a first direction while signal and power supply lines 4 and 6, formed apart from each other and crossing the scan line 2, are formed in a second direction. The scan line 2 together with the signal and power supply lines 4 and 6 define a pixel region. A switching thin film transistor TS, such as an addressing element, is formed near an intersection of the scan and signal lines 2 and 4, and a storage capacitor CST is connected to the switching thin film transistor TS. A driving thin film transistor TD, such as a current source element, is connected to the switching thin film transistor TS, the storage capacitor CST and the power supply line 6. The driving thin film transistor TD is electrically connected to an anode electrode of an organic electroluminescent diode E that is driven by a static current. The anode electrode as well as the cathode electrode are components of the organic electroluminescent diode E. The switching thin film transistor TS serves to control a voltage applied to the driving thin film transistor TS and the storage capacitor CST serves to store a charge to maintain the voltage applied to the driving thin film transistor TS. The driving principle of the related art organic electroluminescent device will be described hereinafter.
When the switching thin film transistor TS is turned on, a data signal can be applied to the driving thin film transistor TD and the storage capacitor CST via the switching thin film transistor TS. When the driving thin film transistor TD is turned on, a current from the power supply line 6 can be applied to the organic electroluminescent diode E. The current passing through the driving thin film transistor TD then passes through the organic electroluminescent diode E such that the organic electroluminescent diode E can emit light. Because the degree in which driving thin film transistor TD is open depends on amplitude of the data signal, gray levels can be displayed by controlling an amount of current that passes through the driving thin film transistor TD. A data signal that is stored in the storage capacitor CST is continuously applied to the driving thin film transistor TD and accordingly the organic electroluminescent diode E can continuously emit light at a specified gray level until a signal for a next frame is applied.
FIG. 2 is a plan view of a pixel of a related art active matrix organic electroluminescent device. As shown FIG. 2, the pixel includes a switching thin film transistor TS and a driving thin film transistor TD. As also shown in FIG. 2, a gate line 37 is formed in a first direction, and a data line 51 and a power supply line 41, which are spaced apart from each other, are formed in a second direction. The gate line 37 crosses the data line 51 and the power supply line 41 and defines a pixel region P between the gate line 37, the data line 51 and the power supply line 41. A switching thin film transistor TS is formed near an intersection of the gate and the data lines 37 and 51 and a driving thin film transistor TD is formed near an intersection of the switching thin film transistor TS and the power supply line 41. The power supply line 41 and a capacitor electrode 34, which is connected to a semiconductor layer 31 of the switching thin film transistor TS, form a storage capacitor CST. A first electrode 58 is electrically connected to the driving thin film transistor TD. Although not shown in FIG. 2, an organic light emitting layer and a second electrode are sequentially formed over the first electrode 58. The area where the first electrode 58 is formed is defined as an organic light-emitting region L. The driving thin film transistor TD has a semiconductor layer 32 and a gate electrode 38. The switching thin film transistor TS has a gate electrode 35. Layered structures of the organic light-emitting region L, the driving thin film transistor TD and the storage capacitor CST will be described hereinafter with reference to FIG. 3.
FIG. 3 is a cross-sectional view along the line III-III′ of FIG. 2. In FIG. 3, a driving thin film transistor TD having a semiconductor layer 32, a gate electrode 38, and source and drain electrodes 50 and 52 is formed on an insulating substrate 1. A power electrode 42 extending from a power supply line (not shown) is electrically connected to the source electrode 50 and a first electrode formed of transparent conductive material is electrically connected to the drain electrode 52. A capacitor electrode 34 is formed under the power electrode 42 of the same material used to form the semiconductor layer 32. The power electrode 42 and the capacitor electrode 34 form a storage capacitor CST. An organic light-emitting layer 64 and a cathode electrode 66 are sequentially formed on the first electrode 58 to complete formation of an organic light-emitting region L. A buffer layer 30 is formed between the substrate 1 and the semiconductor layer 32. First, second, third and fourth passivation layers 40, 44, 54 and 60 respectively having a contact hole for electrical contacting in each layer are formed on the substrate 1. The first passivation layer 40 is formed between the storage electrode 34 and the power electrode 42 and serves as a dielectric substance. The second passivation layer 44 is formed on the power electrode 42 and the third passivation layer 54 is formed between the source electrode 50 and the first electrode 58. The fourth passivation layer 60 is formed between the driving thin film transistor TD and a second electrode 66.
FIGS. 4A to 4I are cross-sectional views illustrating processes of fabricating the related art active matrix organic electroluminescent device, and correspond to cross-sections along the line III-III′ of FIG. 2. The organic electroluminescent device may be manufactured using a photolithographic process in which thin films are patterned by exposing and then developing a photoresist, such as a photosensitive material. As shown in FIG. 4A, a buffer layer 30 is formed on an insulating substrate 1 with a first insulating material. Then, a semiconductor layer 32 and a capacitor electrode 34 are formed of polycrystalline silicon on the buffer layer 30 through a first photolithography process using a first mask.
Subsequently as shown in FIG. 4B, a gate insulating layer 36 and a gate electrode 38 are formed on the semiconductor layer 32 by sequentially depositing a second insulating material and a first metal material on the semiconductor layer 32 and then patterning the deposited material through a second photolithography process using a second mask. Then, as shown in FIG. 4C, a first passivation layer 40 is formed on the whole substrate on which the gate insulating layer 36 and the gate electrode 38 are already formed. A power electrode 42 is subsequently formed on the first passivation layer 40 in an area corresponding to the capacitor electrode 34 by depositing a second metal material on the first passivation layer 40 and then patterning it through a third photolithography process using a third mask.
As shown in FIG. 4D, a second passivation layer 44 having first and second contact holes 46a and 46b and a capacitor contact hole 48 is formed in the first passivation layer 40 by depositing a third insulating material on the first passivation layer 40 and then patterning it through a fourth photolithography process using a fourth mask. The first and second contact holes 46a and 46b expose portions at both sides of the semiconductor layer 32 and the capacitor contact hole 48 exposes a portion of the power electrode 42. The semiconductor layer 32 is doped with impurities such as boron (B) or phosphorus (P) to define a channel region 32a corresponding to the gate electrode 38, and source and drain regions 32b and 32c at both sides of the channel region 32a. 
As shown in FIG. 4E, source and drain electrodes 50 and 52 are subsequently formed on the second passivation layer 44 by depositing a third metal material and then patterning it through a fifth photolithography process using a fifth mask. The source electrode 50 is connected to the source region 32b through the first contact hole 46a and is connected to the power electrode 42 via the capacitor contact hole 48. The drain electrode 52 is connected to the drain region 32c via the second contact hole 46b. The semiconductor layer 32, the gate electrode 38 and the source and drain electrodes 50 and 52 form a driving thin film transistor TD. The power electrode 42 and the capacitor electrode 34 are electrically connected to the source electrode 52 and a semiconductor layer (not shown) of a switching thin film transistor (not shown), respectively and form a storage capacitor CST using the first passivation layer 40 as a dielectric substance.
Referring to FIG. 4F, a third passivation layer 54 having a drain contact hole 56 is formed by depositing a fourth insulating material over the whole substrate 1 on which the source and drain electrodes 50 and 52 are already formed and then patterning it through a sixth photolithography process using a sixth mask. As shown in FIG. 4G, a first electrode 58 is then formed on the third passivation layer 54 in an area corresponding to an organic light-emitting region L by depositing a fourth metal material on the third passivation layer 54 and then patterning it through a seventh photolithography process using a seventh mask. The first electrode 58 is connected to the drain electrode 52 through the drain contact hole 56.
Then, as shown in FIG. 4H, a fourth passivation layer 60 having a first electrode exposure portion 62 exposes a first electrode portion corresponding to the organic light-emitting region L formed by depositing a fifth insulating material over the whole substrate 1 on which the first electrode 58 is already formed and then patterning it through an eighth photolithography process using an eighth mask. The fourth passivation layer 60 also serves to protect the driving thin film transistor TD from moisture and contaminants.
As shown in FIG. 4I, an organic light-emitting layer 64 contacting the first electrode 58 via the first electrode exposure portion 62 is formed on the substrate 1 over which the fourth passivation layer 60 is already formed. A second electrode 66 is subsequently formed on the organic light-emitting layer 64 and the fourth passivation layer 60 by depositing a fifth metal material over the whole substrate 1. If the first electrode 58 is used as an anode electrode, the fifth metal material should have a reflection property so as to reflect the light emitted from the organic light-emitting layer 64 and thus to display an image. In addition, the fifth metal material is selected from the metal materials having a low work function such that the second electrode 66 can easily give away electrons.
FIG. 5 is a cross-sectional view of a related art organic electroluminescent device. As shown in FIG. 5, a first substrate 70 on which a plurality of sub-pixels Psub are defined and a second substrate 90 are spaced apart from each other. An array element layer 80 having a plurality of driving thin film transistors TD corresponding to each sub-pixel Psub is formed on the first substrate 70. A plurality of first electrodes 72 corresponding to each sub-pixel Psub is formed on the array element layer 80. The plurality of first electrodes 72 respectively connects to the driving thin film transistors TD of the sub-pixels Psub An organic light-emitting layer 74 for emitting light of either red (R), green (G) and blue (B) colors from the sub-pixels Psub is formed on the first electrode 72. A second electrode 76 is formed on the organic light-emitting layer 74. The first and second electrodes 72 and 76, and the organic light-emitting layer 74 form an organic electroluminescent diode E. Light emitted from the organic light-emitting layer 74 passes through the first electrode 72. That is, the organic electroluminescent device is a bottom emission-type organic electroluminescent device.
The second substrate 90 is used as an encapsulating substrate and has a depressed portion 92 at a middle surface thereof and a moisture absorbent desiccant 94 for protecting the organic electroluminescent diode E from exterior moisture. The second substrate 90 is spaced apart from the second electrode 76 at a certain distance. A seal pattern 85 is formed on one of the first and second substrates 70 and 90 and later used to attach the first and second substrates 70 and 90.
Attaching the substrate having the array element layer and the organic electroluminescent diode to an additional encapsulating substrate completes the related art bottom emission-type organic electroluminescent device. If the array element layer and the organic electroluminescent diode are formed on the same substrate, then yield of a panel having the array element layer and the organic electroluminescent diode is dependent upon the individual yields of the array element layer and the organic electroluminescent diode. The yield of the panel is greatly affected by the yield of the organic electroluminescent diode. Accordingly, if an inferior organic electroluminescent diode that is usually formed of a thin film having a thickness of 1000 Å has a defect due to impurities and contaminants, the panel is classified as a defective panel. This leads to wasted production costs and material, thereby decreasing yield.
Bottom emission-type organic electroluminescent devices have the advantages of high image stability and variable fabrication processing. However, the bottom emission-type organic electroluminescent devices are not adequate for implementation in devices that require high resolution because bottom emission-type organic electroluminescent devices have a small aperture ratio due to the thin film transistor blocking some of the light transmission. Top emission-type organic electroluminescent devices have a large aperature ratio. In addition, since top emission-type organic electroluminescent devices emit light upward of the substrate, the light can be emitted without being blocked by the thin film transistor that is positioned under the light-emitting layer. Accordingly, design of the thin film transistor may be simplified in top emission-type organic electroluminescent device. In addition, the aperture ratio can be increased, thereby increasing operational life span of the organic electroluminescent device. However, since a cathode is commonly formed over the organic light-emitting layer in the top emission-type organic electroluminescent devices, material selection and light transmittance are limited such that light transmission efficiency is lowered. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film passivation layer may fail to prevent infiltration of exterior air into the device.